1. Technical Field
The present invention relates to a nonaqueous electrolyte. Particularly, the present invention relates to a nonaqueous electrolyte secondary battery in which, without lowering of the battery capacity, thermal stability and cycle property at higher temperatures have been remarkably improved.
2. Related Art
With the rapid spread of portable electronic equipment, the specifications required of the batteries used in such equipment have become more stringent with every year, and there is particular requirement for batteries that are compact and thin, have high capacity and superior cycling characteristics, and give stable performance. In the field of secondary batteries, attention is focusing on lithium nonaqueous electrolyte secondary batteries, which have high energy density compared with other batteries. These lithium nonaqueous electrolyte secondary batteries are winning an increasingly large share of the secondary battery market.
FIG. 1 is a perspective view showing a cylindrical nonaqueous electrolyte secondary battery produced conventionally by sectioning the battery perpendicularly. A nonaqueous electrolyte secondary battery 10 uses a coil-shaped electrode body 14 produced by winding a positive electrode 11, a separator 13 and a negative electrode 12 which are laminated in this order, and is produced by a method including: disposing insulating plates 15 and 16 respectively on the top side and bottom side of the coil-shaped electrode body 14 to prepare a parts set; holding the parts set in the inside of a steel-made cylindrical battery outer packaging can 17 serving also as a negative electrode terminal; welding not only a power collecting tab 12a of the negative electrode 12 to an inside bottom of the battery outer packaging can 17, but also a power collecting tab 11a of the positive electrode 11 to a bottom plate of a current-intercepting opening-sealing body 18 with a built-in safety device; pouring a predetermined nonaqueous electrolyte through an opening of the battery outer packaging can 17; and sealing the battery outer packaging can 17 with the current-intercepting opening-sealing body 18. Such a nonaqueous electrolyte secondary battery has such an excellent effect that battery performance and reliability are high.
As a negative electrode active material used in the nonaqueous electrolyte secondary battery, carbonaceous materials such as graphite and an amorphous carbon are widely used, since carbonaceous materials have high safety because dendrites do not grow therein due to their discharge potential with lithium metal or lithium alloy, and also such excellent properties as excellent initial efficiency, advantageous potential flatness and high density.
Further, as a nonaqueous solvent of a nonaqueous electrolyte, carbonates, lactones, ethers and esters are used individually or in combination of two or more thereof. Among them, particularly carbonates having a large dielectric constant and having a large ion conductivity as the nonaqueous electrolyte thereof are frequently used.
It is known that as a positive electrode active material, when a lithium compound oxide such as lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMnO2), spinel-type lithium manganese oxide (LiMn2O4), lithium nickel oxide (LiNiO2) and lithium iron oxide (LiFeO2) is used in combination with a negative electrode consisting of a carbon material, a 4-V-class nonaqueous secondary battery having a high energy density can be obtained. Among them, particularly because of various battery properties more excellent than those of other materials, lithium cobalt oxide is frequently used. However, since cobalt is not only expensive, but also the existing amount of cobalt as a resource is small, for continued use of lithium cobalt oxide as a positive electrode active material of a nonaqueous electrolyte secondary battery, it is desired to make the nonaqueous electrolyte secondary battery having even higher performance and longer life.
For making the nonaqueous electrolyte secondary battery in which lithium cobalt oxide is used as a positive electrode active material, having even higher performance and longer life, it is essential to enlarge the capacity of the battery and improve the cycle life of the battery. Since lithium cobalt oxide as a positive electrode active material is exposed to an electric potential of 4 V or more based on lithium during charging the battery, when the charging-discharging cycle is repeated many times, cobalt in lithium cobalt oxide is dissolved out and the battery is deteriorated, so that the loading performance thereof is lowered and the discharging capacity is also lowered. Thus, during the synthesis of LiCoO2 as a positive electrode active material, another transition element M is added and contained, so that different metal element-added lithium cobalt oxide represented by a general formula: LiCo1-xMxO2 has been employed. With respect to the different metal element-added lithium cobalt oxide represented by a general formula: LiCo1-xMxO2, since during the use thereof, the dissolution of cobalt is suppressed, various battery properties compared to those in the case where lithium cobalt oxide is used individually, have been achieved.
It is also known that when magnesium is used as this different metal element, particularly the thermal stability becomes excellent. For example, JP-A-4-171659 describes that by using lithium cobalt oxide to which magnesium is added as a positive electrode active material, a nonaqueous electrolyte secondary battery in which cycle property and charged state property at higher temperatures have been improved, is obtained. This magnesium-added lithium cobalt oxide is produced by a method including: mixing thoroughly lithium carbonate and magnesium carbonate which are weighed so that atom ratios of lithium and magnesium relative one atom of cobalt are respectively 0.5 to 1.0:0.5 to 0; and hydrolyzing the resultant mixture.
Further, JP-A-2002-198051 describes that by using as a positive electrode active material, different metal element-added and coprecipitated cobalt oxide obtained by coprecipitating a lithium compound and an added element M (wherein, the added element M is at least one selected from Mg, Al, Cu and Zn), a nonaqueous electrolyte secondary battery having a high active material specific capacity and excellent charging and discharging cycle property and capable of suppressing the increase of the battery thickness, can be obtained. Also, JP-A-2003-45426 describes that by using as a positive electrode active material, different metal element-added lithium cobalt oxide represented by a general formula: LixCoyMzO2 (wherein, M is at least one element selected from Mg, Al, Si, Ti, Zn, Zr and Sn), the phase transition is suppressed and a degradation of the crystal structure is caused a little, so that not only while maintaining a high capacity, the thermal stability during the charging is improved, but also satisfactory charging-discharging property can be achieved.
Further, JP-A-2004-47437 describes that by using as a positive electrode active material, a material composed of particles of a compound oxide containing lithium and cobalt which is composed of an element M1 selected from the group consisting of Mg, Cu and Zn and an element M2 selected from the group consisting of Al, Ca, Ba, Sr, Y and Zr, wherein the element M1 is distributed uniformly in the above particles and the element M2 is distributed in the above particles more in the inside part than in the surface layer part, a nonaqueous electrolyte secondary battery in which without decreasing the tap density of the positive electrode active material, the improvement of the cycle property and the thermal stability can be achieved, can be obtained.
As described above, it is known that a nonaqueous electrolyte secondary battery using different metal element-added lithium cobalt oxide in which Mg or the like as a different metal element is homogeneously added by a coprecipitation during the synthesis of lithium cobalt oxide as a positive electrode active material exhibits excellent cycle property and thermal stability compared to a nonaqueous electrolyte secondary battery using lithium cobalt oxide individually. However, the more the adding amount of the different metal element in the different metal element-added lithium cobalt oxide is, not only is the battery capacity lower, but the cycle property of a nonaqueous electrolyte secondary battery is also lower. In contrast, by reducing the adding amount of the different metal element in the different metal element-added lithium cobalt oxide and by increasing the content of lithium cobalt oxide, making a nonaqueous electrolyte secondary battery having a high capacity can be just achieved; however, the cycle property and the thermal stability are lowered. Therefore, in a conventional nonaqueous electrolyte secondary battery, when a different metal element is added to lithium cobalt oxide as a positive electrode active material, it is difficult to achieve a balance between the improving effect of the cycle property, the thermal stability and the like and the enlarging of the battery capacity.